The invention relates to an improved nickel-chromium-molybdenum-niobium (NiCrMoNb) alloy especially suitable for turbine cover buckets.
In U.S. Pat. Nos. 5,509,784, 7,270,518 and 7,344359, a plurality of steep angle bucket covers are disclosed. The covers are integral with the airfoils of the buckets and the buckets, are mounted in a circumferential array about a turbine wheel. The bucket covers include forward and aft clearance surfaces which extend generally parallel to the axis of rotation of the turbine rotor and lie on opposite sides of the airfoil of the bucket. Intermediate the clearance surfaces are contact surfaces and a radii. It will be appreciated that the adjacent covers on the opposite sides of each bucket include substantially complementary shaped cover edges whereby the clearance surfaces are circumferentially spaced from one another and the contact surfaces contact one another during turbine operation. The contact surfaces of the adjoining covers have interference fits which cause and maintain a coupling between the covers during operation. That is, the covers are biased such that the contact surfaces of the adjoining covers are maintained in contact with one another. This, however, applies a stress to the covers which has the potential to cause high cycle fatigue cracks along the covers. Analysis of the potential problem has indicated that the high cycle fatigue cracks are a function of fretting fatigue on the pressure side of the cover's contact surface. The cracks are initiated on the pressure side contact surface at a location adjacent the inner corner radius between the clearance surfaces where the mating suction side cover contact surface separates from the pressure side contact surface.
U.S. Pat. Nos. 3,046,108 and 3,160,500 disclose nickel chromium alloys having certain advantageous properties. These alloys are referred to as alloy 625. Alloy 625 is not used in certain high temperature applications because it lacks the necessary yield strength.